The $787 billion economic stimulus package President Obama signed earlier this year is likely to cost “tens of billions of dollars” more than expected, helping to drive projections for next year’s budget deficit to $1.5 trillion, White House budget director Peter Orszag told reporters.

With unemployment climbing, costs for a variety of stimulus programs are running higher than anticipated, Orszag said, including expanded unemployment benefits, food stamps and energy grants. In an interview embargoed for release Tuesday morning, Orszag said he could not estimate the overall cost of the package, but he called Republican estimates of $900 billion “slightly high.”

The CBO also addresses this issue in its report (box on pp. 10-11). The box discusses lots of pesky nuances about budget accounting and the timing of payments. Perhaps the most interesting observation, consistent with the OMB quote above, is that:

The higher-than-expected unemployment rate has led CBO to raise its estimates of spending in 2009 for ARRA [i.e., stimulus] provisions that affect unemployment compensation (by $7 billion) and Medicaid (by $1 billion).

In other words, the weaker economy has added $8 billion to stimulus spending in fiscal 2009 alone with, presumably, more to come in fiscal 2010.